Eight fields (viz. Suppl. 1,...,Supp. 4 and Addit.text 1 to Addit.text 4) have already been provided by default by SAP on Addit. Data tab of VT01N/VT02N/VT03N. You can transform and use them as per your need.

Press F1 on any of these fields present on Addit. Data tab and follow the instructions.

If you have any specific requirement to add more custom fields, please refer to below threads: